Document `from_fd` net.box module function

Product: Tarantool
Since: 3.0
Root document: https://www.tarantool.io/en/doc/latest/reference/reference_lua/net_box/
SME: @ locker

Details

The function takes a file descriptor number (fd) as its first argument
and a table of connection options as its second, optional argument. It
creates and returns a new connection object.

The fd should point to a socket and be switched to the non-blocking
mode, but this isn't enforced. If the fd is invalid, the connection may
not work as expected.

The functions take the same options as connect.

The function takes the ownership of the fd, i.e. the fd must not be used
or closed after this function successfully returns.

Example:

The code below connects to a Tarantool instance at port 3301 using
the socket module, then wraps the socket fd in a net.box
connection object.

local net = require('net.box')
local socket = require('socket')

-- Connect a socket then wrap it in a net.box connection object.
local s = socket.tcp_connect('localhost', 3301)
assert(s ~= nil)
local conn = net.from_fd(s:fd(), {fetch_schema = false})
s:detach()

conn:call('box.schema.user.info')
conn:close()

Requested by @locker in https://github.com/tarantool/tarantool/commit/ae441c995e25c965a969ac1381487fc23c1f0643.

Update the net.box reference page linked as the root document, locating the module-function section and the existing connect documentation. Document from_fd's arguments, non-blocking fd requirement, option handling, ownership, and the provided socket example; done when the function is covered consistently in the reference page.